Pressure sensor with fibre-integrated bragg grating, comprising an integrated temperature sensor with fibre-integrated bragg grating

ABSTRACT

The subject-matter of the present invention is a wavelength-coded fiber Bragg grating pressure sensor  1  which is suitable, in particular, for use in the case of high pressures and temperatures in oil drill holes. The sensor principle according to the invention is based on the fact that the hydrostatic pressure of a liquid or gaseous medium  11  is converted with the aid of a transducer  1  into a longitudinal fiber elongation or fiber compression. The transducer  1  comprises a measuring or pressure cylinder  7   a  which exchanges pressure with the medium  11,  and a reference cylinder  7   b  which is shielded from the medium  11  or oppositely pressure-loaded. Temperature-compensated transducers  1  with a temperature-independent Bragg wavelength λ B  can be realized by introducing a suitable temperature dependence of the mechanical prestressing of the pressure sensor fiber  3  by selecting the materials, lengths and arrangements of the fiber holder supports  5   a,    5   b.  A fiber Bragg grating temperature sensor  19, 20  can be provided, in addition. Also specified are transducers  1  with other cylinder arrangements  7   a,    7   b,  and a wavelength-division multiplex configuration with several transducers  1.

DESCRIPTION Fiber Bragg grating pressure sensor with integrated fiberBragg grating temperature sensor TECHNICAL FIELD

The invention relates to the field of fiber-optic pressure andtemperature measurement.

PRIOR ART

In oil production, drill holes have to be monitored with regard topressure and temperature. The liquid pressures in the drill hole can beup to 100 NPa (1000 bar) , and the temperatures can be up to over 200°C. Electric sensors such as, for example, piezoelectric resistors,piezoelectric elements, capacitive probes, or crystal resonators arefrequently used in pressure measurement up to approximately 170° C. Itis also known to use optical pressure sensors which are distinguished bygood high temperature capacity, corrosion resistance and insensitivityto electromagnetic interference. Examples of this are mechanicalresonators, which are activated optically and read out optically,elastooptic sensors, optical sensors with a pressure-sensitivediaphragm, or Fabry- Perot resonators.

A further optical sensor with fiber Bragg gratings for measuringmaterial elongations is disclosed, for example, in U.S. Pat. No.4,761,073. A refractive index grating, which is written by UV light intoa monomode fiber, acts as a reflector or transmission filter with acharacteristic Bragg wavelength λ_(B). Longitudinal fiber elongationschange the grating period and refractive index and displace the Braggwavelength λ_(B). The output signals are wavelength-coded andindependent of the received light power. Serial multiplexing of severalelongation sensors can be realized very easily by writing in along asensor fiber several Bragg gratings with different reflectionwavelengths whose signals can be separated spectrally. It is proposed toeliminate signal interference based on thermal grating elongations withthe aid of superimposed gratings of different reflection wavelengths. Itis known that instead of being done in a wavelength-selective fashion itis also possible for multiplexing to be performed by time-resolvedmeasurements with the aid of a pulsed light source. In order to monitorbody deformations, the sensor fiber is typically fastened on the surfaceof the body or embedded in the body. If Bragg gratings are used forelongation measurements, the measurement range is limited by theultimate fiber strength.

Fiber Bragg grating sensors for measuring isotropic pressures of liquidsare presented in the article by M.G. Xu et al., “Optical In-FibreGrating High Pressure Sensor”, Electronics Letters 29 (4), 398-399(1993). The sensor fiber is introduced with the Bragg grating into ahigh-pressure vessel and immediately exposed to the hydrostatic pressureof a fluid. However, the isotropic pressure sensitivity is exceptionallylow for Bragg gratings in glass fibers; the specific Bragg wavelengthdisplacement is typically only 0.0003 nm/ 100 kPa at 1550 nm. Moreover,it is necessary to compensate temperature effects because of the hightemperature sensitivity of typically 0.01 nm/°C.

An apparatus for longitudinal compression of optical fibers is describedin U.S. Pat. No. 5,469,520. The sensor fiber is threaded with the fiberBragg grating into several cylindrical ferrules and two end tubelets,and the ferrules and tubelets are mounted in a groove between two metalblocks which can be screwed to one another. The ferrules can bedisplaced laterally in the groove, one tubelet is connected to the metalblocks and the other is fastened on a moveable slide. By displacing theslide, the fiber is compressed on the free links between the tubelets,in particular between the ferrules, and lateral escape is simultaneouslyprevented by the groove. It is possible to realize a very wide pressuremeasurement range because of the fact that the pressure strength ofglass fibers (“fused silica fibers”) is approximately 20 times greaterthan their elongation strength.

U.S. Pat. No. 5,042,898 discloses a device for temperature stabilizationof fiber Bragg gratings. The fiber Bragg grating is clamped over a gapbetween two supports with different coefficients of thermal expansion.The supports are screwed to one another at a common supporting point viaa spacer thread with the aid of which the gap width and/or fiberprestressing and/or Bragg wavelength can be set. The differential fiberelongation between the fiber holders is dimensioned precisely such thatthe thermally induced changes in the Bragg wavelength can becompensated. This is achieved by selecting the support materials and thespacings between the supporting point and the fiber holders. In anembodiment which can be subjected to pressure, a glass capillary isprovided in the gap for the purpose of accommodating the fiber Bragggrating. The carrier materials and the length and inside and outsidediameters of the glass capillary are to be coordinated with one anotherfor the purpose of temperature compensation. A temperature-stabilizedfiber Bragg grating of this type can be used as a wavelength standard,for stabilizing the emission wavelength of laser diodes, or as awavelength filter in fiber-optic sensors.

SUMMARY OF THE INVENTION

It is the object of the present invention to specify a fiber Bragggrating pressure sensor which is suitable for wavelength-codedmeasurement of isotropic pressures in liquids or gases and isdistinguished by a compact transducer which can be designed for highpressures. This object is achieved according to the invention by meansof the features of claim 1.

Specifically, the core of the invention is to specify a fiber-optictransducer in which a pressure sensor fiber with a fiber Bragg gratingis fastened on supports by means of fiber holders, and at least onesupport is fitted with a pressure member for converting an all roundpressure of a medium into a longitudinal elongation of the pressuresensor fiber.

A first exemplary embodiment shows a pressure-transmitting element(=transducer) with a pressure-loaded inner cylinder and an unloadedouter cylinder, which are arranged in a housing whose differentialpressure elongation is transmitted to a sensor fiber, and whosedifferential temperature elongation stabilized the Bragg wavelength ofthe sensor fiber.

A second exemplary embodiment relates to variants of the transducer inthe case of which the outer cylinder is simultaneously the housing andis subjected to pressure, and in the case of which the sensor fiber canalso be placed under compressive load.

A third exemplary embodiment constitutes a transducer with an annularcylinder which is pressure loaded from inside, and force-transmittingcenter cylinders for elongation of the sensor fiber at both ends.

A fourth exemplary embodiment constitutes a transducer with a supportingcylinder which is pressure loaded from outside, and center cylinders,which are oppositely pressure-loaded for the purpose of relievingpressure from the sensor fiber at both ends.

Further exemplary embodiments relate to fiber holders and ferrules forfixing and prestressing the pressure sensor fiber in the transducer.

Another exemplary embodiment constitutes a serial, reflexivemultiplexing arrangement of several fiber Bragg grating pressure sensorswith different Bragg wavelengths, which are fed via a common broadbandlight source and are detected in a wavelength-selective fashion.

Additional exemplary embodiments follow by combining features which areessential to the invention, and from the dependent claims.

An important advantage of the fiber Bragg grating pressure sensoraccording to the invention consists in that it is possible with the aidof the wavelength-coded pressure signal to realize a high measuringaccuracy, a wide pressure measurement range of up to 100 MBa and a largemeasuring distance between the passive sensor head and active opticalsystem and electronic system.

A further advantage of the fiber Bragg grating pressure sensor consistsin that it is possible for the temperature sensitivity to be largelysuppressed by a differential design of the transducer, and thereby forthe reliability of (quasi)static pressure measurements to besubstantially improved.

Finally, also very advantageous features of the fiber Bragg gratingpressure sensor are the ease with which it can be combined with a fiberBragg grating temperature sensor, the simple ability to multiplexseveral combined pressure and temperature sensors and, overall, theexcellent suitability for use under high pressures, at high temperaturesand with strong corrosion, in particular in oil drill holes.

BRIEF DESCRIPTION OF THE DRAWING

The invention is explained below with the aid of exemplary embodiments.For a fiber-optic temperature-compensated pressure sensor with anintegrated temperature sensor:

FIGS. 1a, 1 b show a mechanical pressure transmitting element(=transducer) with an inner cylinder which is pressure loaded, and anunloaded outer cylinder for differential elongation of the sensor fiber;

FIGS. 2a, 2 b show a transducer with inner and outer cylinders which areoppositely pressure-loaded, for pressure relief (2 a) or for compression(2 b) of the sensor fiber;

FIG. 3a shows a transducer with a hollow cylinder which is pressureloaded from inside, and force transmitting center cylinders forelongating the sensor fiber at both ends;

FIG. 3b shows a transducer with a hollow cylinder which is pressureloaded from outside, and center cylinders, which are oppositely pressureloaded, for pressure relief of the sensor fiber at both ends;

FIGS. 4a-4 e show fiber holders and ferrules: (4 a) a spliced-on glasscapillary, (4 b) a glass solder connection, (4 c) a clamping ferrule, (4d) a spliced-in fiber with thick cladding; and (4 e) a fastening of theferrules on the transducer; and

FIG. 5 shows a reflecting arrangement of several wavelength-divisionmultiplex pressure and temperature sensors.

Identical parts are provided with identical reference symbols in thefigures.

WAYS OF IMPLEMENTING THE INVENTION

FIGS. 1a, 1 b show a first embodiment of the sensor 1 orpressure-transmitting element V transducer) 1 schematically in crosssection. The transducer 1 includes an optical fiber 2, which has twosections with a pressure sensor fiber 3 and an optional temperaturesensor fiber 19 respectively having one written-in fiber Bragg grating4, 20 each. The pressure sensor fiber 3 is held between two fiber orferrule holders 6 a, 6 b and prestressed. The holders 6 a, 6 b arepermanently connected via end plates 8 a, 8 b to supports 5 a, 5 b suchthat their pressure and temperature-induced elongations can betransmitted to the pressure sensor fiber 3.

A first support 5 a comprises as measuring or pressure cylinder 7 a aninner cylinder 7 a which can be extended by internal pressure and whichhas an opening 18 in a transducer wall 14 and a cavity 9 with a plungerface 8 a. The opening 18 serves to pick up the pressure of a medium 11,the cavity 9 serves to transmit pressure by means of a fluid 10, and theplunger face 8 a serves to output pressure to a fiber holder 6 a. Thefluid 10 is normally the medium 11 itself. In the case of all thetransducer design [sic] (FIGS. 1a-3 b), the opening 18 can be selectedto be small and, possibly, fitted with a pressure compensating element29, for example a diaphragm or a bellows, and a silicone oil or the likecan be provided as the fluid 10, which is preferably not verycompressible. The inner cylinder 7 a is connected in a pressure-tightfashion to the wall 14 in the region of the opening 18. Thepressure-insulated housing interior 21 is advantageously filled with alow-pressure gas, for example dry nitrogen, at a few 100 mbar. As aresult, adequate heat transfer is ensured between the medium 11 ortransducer 1 and the temperature-sensitive fiber Bragg grating 20, andat the same time the influence of temperature-induced gas pressurechanges in the housing interior is kept small. In less demandingapplications, tilling at normal pressure, or a vacuum can suffice. Asecond support 5 b comprises a reference cylinder or outer cylinder 7 bwhich is shielded from the ambient medium 11 and can therefore not beloaded, and which is not in contact with the fluid 10. The cylinders 7a, 7 b are fastened on the transducer wall 14 and encapsulated in atransducer housing 12. The housing 12 preferably comprises a housingcylinder 13 with walls 14, 15 into which pressure-tight fiber bushings16, 17 are inserted. Clamps or the like (not illustrated) can beprovided to terminate fiber cables outside at the walls 14, 15.

The pressure and temperature sensitivity of a sensor 1 in accordancewith figures 1 a, 1 b can be adapted and, in particular, optimized bydimensioning the transducer 1 in a fashion according to the invention.The pressure-induced elongation ΔL₁ of the inner cylinder 7 a of lengthL₁ is

ΔL₁=ε_(t) L₁  (G1a)

ε_(t), that is to say the longitudinal extension of the inner cylinder 7a, being calculated from

ε_(t)=(1/E)·[σ_(x)−μ(σ_(ω)+σ_(r))]  (G1b)

ε_(t)=(1/E)·[pR_(i) ²/(R₁ ²−R_(i) ²)]·[1−2μ]  (G1c)

with inside radius R_(i), outside radius R_(a), Young's modulus ofelasticity E, Poisson's ratio μ and pressure p of the medium 11 to bemeasured.

In this case, σ_(x) is the longitudinal, σ_(r)(r) the radial andσ_(ω)(r) the tangential stress of the inner cylinder 7 a:

σ_(x)=pR_(i) ²/(R_(a) ²−R_(i) ²)  (G1d)

σ_(r)(r)=[pR_(i) ²−pR_(i) ² R_(a) ²/r²]/[(R_(a) ²−R_(i) ²)  (G1e)

σ_(ω)(r)=]pR_(i) ²+pR_(i) ²R_(a) ²/r²]/(R_(a) ²−R_(i) ²)  (G1f)

The length L₂ of the outer cylinder 7 b remains unchanged, with theresult that the pressure sensor fiber 3 of length 1 experiences anelongation of

Δ1=ΔL₁  (G2).

Glass fibers (“fused silica fibers”) exhibit a linearly elastic,hysteresis-free behavior for relative elongations Δ1/1 up toapproximately 0.01. The displacement of the Bragg wavelength is alsoproportional to the elongation in this range. On the other hand, thelinear elasticity range for steel as a typical material for an innercylinder 7 a is limited to relative extensions of ΔL₁/L₁≦0.001.

For a wide pressure-measurement range of high resolution, thepermissible elongation of the fiber should be utilized to the greatestpossible extent. On the other hand, a not too high elongation favors along service life of the fiber. A maximum fiber elongation of Δ1/1=0.005 is a sensible compromise between high resolution and long servicelife. The relationship then holds, with p_(max)=maximum pressure, theextension limit ΔL₁/L₁≦0.001 of the inner cylinder 7 a or transducer 1requiring to be observed. A quantitative example for this: p_(max)=100MPa, ε=1.96·10¹¹ Pa, μ=0.28, R_(a)=4 mm, R_(i)=2.8 mm. The length ratioof inner cylinder 7 a to pressure sensor fiber 3, or the extensiontransmission ratio L₁/1=23 then requires to be selected, for example1=15 mm and L₁=34.5 cm. For the given parameters, the deformationcorresponding to the longitudinal stress σ_(x) of the inner cylinder 7 ais 0.5·10⁻³. The deformation corresponding to the tangential stress is$\begin{matrix}\begin{matrix}{{\Delta \quad {L_{t}/L_{t}}} = {{{\sigma_{\omega}\left( R_{a} \right)}/E} = {2 \cdot \left( {p/E} \right) \cdot {R_{i}^{2}/\left( {R_{a}^{2} - R_{i}^{2}} \right)}}}} \\{= {1.0 \cdot 10^{- 3}}}\end{matrix} & ({G4a})\end{matrix}$

on the outside (r=R_(a)) of the inner cylinder, and $\begin{matrix}\begin{matrix}{{\Delta \quad {L_{t}/L_{t}}} = {{{\sigma_{\omega}\left( R_{i} \right)}/E} = {2 \cdot \left( {p/E} \right) \cdot {R_{i}^{2}/\left( {R_{a}^{2} - R_{i}^{2}} \right)}}}} \\{= {1.5 \cdot 10^{- 3}}}\end{matrix} & ({G4b})\end{matrix}$

on the inside (r=R_(i))

The deformation corresponding to the radial stress is

ΔL_(r)/L_(r)=σ_(r)(R_(a))/E=0  (G4c)

on the outside of the inner cylinder, and

ΔL_(r)/L_(r)=σ_(r)(R_(i))/E=−(p/E)=0.5·10⁻³  (G4d)

on the inside of the inner cylinder.

The deformations therefore remain in the elastic range.

The invention also relates to a compensation of thermally induced Braggwavelength displacements of the pressure sensor fiber 3 by means of anopposite differential thermal expansion of the cylinders 7 a, 7 b.

In the simplest case in accordance with FIG. 1a, the cylinders 7 a, 7 bare selected from two materials with different coefficients of thermalexpansion α₁ and α₂. The dimensioning of the cylinder lengths L₁ and L₂is performed as in the U.S. Pat. No. 5,042,898 mentioned at thebeginning. The relative Bragg wavelength displacements owing totemperature (Δλ_(B))_(T)λ_(B) and the temperature-induced fiberelongation (Δλ_(B))_(ε)/λ_(B) are intended to compensate one another,that is to say to be of the same size and opposite sign, specifically

(Δλ_(B))_(T)/λ_(B)=−(Δλ_(B))_(ε)/λ_(B).  (G5)

With the aid of the equations

(Δλ_(B))_(T)/λ_(B)=6.67·10⁻⁶°C⁻¹,  (G6)

(Δλ_(B))_(ε)/λ_(B)=0.78·Δε_(T) and  (G7)

Δε_(T)=[(α₁·L₁−α₂·L₂−α_(f)·1)/1]· ΔT,  (G8)

where α_(f)=0.5·10⁻⁶° C⁻¹=thermal elongation of the pressure sensorfiber 3, this yields the dimensioning rule for the cylinders 7 a, 7 b,specifically

(α₂·L₂−α₁·L₁)/1=8.0·10⁻⁶°C⁻¹,  (G9)

where 1=L₁−L₂. Assuming that 1=15 mm, L₁=30 cm and therefore L₂=28.5 cm,for α₁=12.4 ·10⁻⁶°C⁻¹ the required thermal coefficient of expansion ofthe outer cylinder 7 b is yielded as α₂=13.5·10⁻⁶°C⁻¹. In particular,the fiber elongation Δε_(T) must decrease with increasing temperatureand at constant pressure. The prestressing is therefore to be selectedto be so large that the pressure sensor fiber 3 is still undermechanical stress even at the highest operating temperatures andvanishing pressure.

This transducer 1 is distinguished by a high mechanical stability, lowtemperature sensitivity and the ease with which it can he produced. Thecylinders 7 a, 7 b preferably consist of stainless steel. In addition tolinear coefficients of thermal expansion, in accordance with equation(G9) suitable transducer materials are also to have a slightnonlinearity in thermal expansion, and a high corrosion resistance.These requirements are particularly important in the case of sensors foroil drill holes, since a wide temperature range (0°C−230°C) must becovered and there is an extremely high risk of corrosion. Consequently,the selection is limited to steels of a few, extremelycorrosion-resistant types, and in many instances passive temperaturecompensation cannot be carried out, or can be carried out onlyincompletely.

FIG. 1b specifies a variant of the transducer 1 which permits completetemperature compensation even given a limited selection of materials.The idea according to the invention consists in assembling one or bothcylinders 7 a, 7 b from two or more segments of selectable length andwith different coefficients of thermal expansion, and in this way totailor the differential expansion of the cylinders 7 a, 7 b. Inparticular, as before, the inner cylinder 7 a is designed in one piece,and the outer cylinder 7 b in two pieces. For example, the segment 22may consist of the same type of steel as the inner cylinder 7 a. It mayhave a length L₂′ and a coefficient of expansion α₁. Segment 23 mayconsist of another type of steel, have a length of L₂″ and a coefficientof expansion α₂. The new relationships

(α₂·L₂″−α₁·α₁·(L₂″+1)/1=8.0·10⁻⁶°C⁻¹,  (G10)

L₂=L₂′+L₂″  (G11)

hold in this case.

Thus, for given coefficients of expansion α₁, α₂ and pressure sensorfiber length 1, the segment lengths L₂″, L₂′ of the outer cylinder 7 bare to be selected in accordance with equations (G10) (G11) for completetemperature compensation it being the case that the inner cylinderlength L₁=1 +L₂. It is preferably possible to combine a nickel-basedalloy (for example “Hastealloy C-22” from Hynes International withα₁=12.4·10⁻⁶°C⁻¹) with a chromium-nickel steel (for example “AISI 304”with α2 =17.0·10⁻⁶°C⁻¹). L₂′=6.65 cm and L₂″ =21.85 cm are then to beselected in the above example with 1=15 mm and L₁=30 cm.

It is preferable in FIGS. 1a and 1 b, and generally in the case of alldisclosed transducers 1, to provide a temperature sensor fiber 19 with afiber Bragg grating 20 which is not prestressed. Apart from being usedto monitor the ambient temperature, the temperature information which isobtained from the temperature-induced Bragg wavelength displacement canalso serve the purpose of electronic compensation of a residualtemperature dependence of the pressure measurement.

FIG. 2a shows an embodiment of the transducer 1 without encapsulation12. The transducer 1 is designed in an exactly similar way as before,but the outer cylinder 7 b also functions as housing 12. Both the innercylinder 7 a and the outer cylinder 7 b have plunger faces 8 a and 8 bwhich can be loaded from outside with opposite pressure. The volume 21between the cylinders 7 a, 7 b is sealed in a pressure-tight fashion andpreferably filled, as before, with low-pressure gas. The fiber 2 extendsalong the axis of the concentric cylinders 7 a, 7 b and forms in turn apressure sensor fiber 3 and, in particular, a temperature sensor fiber19 with the associated fiber Bragg gratings 4, 20. The fiber holders 6a, 6 b for the pressure sensor fiber 3 are fastened in the center of theplunger faces 8 a, 8 b and can simultaneously be designed aspressure-tight fiber bushings 16, 17. The fiber prestressing is selectedto be large enough to cover the desired pressure and temperature range.The temperature sensor fiber 19 is preferably protected against themedium 11 by a capillary 24 which is held by struts 25, and can befitted at the outer end with fiber cable clamps (not illustrated).

This transducer 1 is distinguished in that the opposite compressiveextension of the plunger faces 8 a, 8 b effects pressure relief of thesensor fiber 3 at both ends, and thus a doubled pressure sensitivity inconjunction with unchanged cylinder dimensions, in particular withconstant ratios of inside to outside radii. Alternatively, the sameBragg wavelength displacement at pressure can be achieved in the case ofhalf the length L₂ of the transducer 1. Moreover, the diameter of thetransducer 1 without housing 12 can be selected to be very small, andthis is of great significance for use in drill holes. In addition, it ispossible in the case of transducers 1 according to FIG. 2a or 2 b toachieve temperature compensation by a tailored differential extension ofthe cylinders 7 a, 7 b in accordance with FIG. 1a or 1 b.

FIG. 2b shows a variant of FIG. 2a, in the case of which the pressuresensor fiber 3 can be loaded by compression. Arranged for this purposein the housing interior 21 is a preferably cylindrical block 26 with abore 27, which block serves the purpose of accommodating the pressuresensor fiber 3 with the fiber Bragg grating 4. The bore hole diameter isselected to be somewhat larger than the fiber diameter such that thefiber can be displaced longitudinally, but cannot escape laterally undercompression. Because of the high compressive strength of the fibers 3,it is possible to realize a wide measurement range and a high resolutionpower. It is possible in principle for any design of the transducer 1according to the invention for which the sensor fiber 3 is relieved ofpressure (FIGS. 2a, 2 b, 3 b) or can be precompressed (FIGS. 1a, 1 b, 3a) to be fitted with such a compression arrangement 26, 27.

FIG. 3a illustrates a further exemplary embodiment of a transducer 1.The two supports 5 a, 5 b for fastening the fiber holders 6 a, 6 b arecombined here to form a common hollow or annular cylinder 7 c andprovide mutual support for one another. The supports 5 a, 5 b thereforesurround as pressure cylinder 7 c the annular cylinder 7 c, which can beextended by internal pressure and separate holders 28 a, 28 b, inparticular symmetrically arranged center cylinders 28 a, 28 b via whichthe fiber holders 6 a, 6 b can be operationally connected to the annularcylinder 7 c. In particular, the annular cylinder 7 c comprises a cavity9 with an opening 18 laterally in the cylinder outer wall and endplunger faces 8 a, 8 b in the form of annular cylinder covers 8 a, 8 b.The annular cylinder 7 c is connected in a pressure-tight fashion to thehousing cylinder 13 in the region of the opening 18 and mounted in thetransducer housing 12 via positioning elements 30. The housing interior21 outside the annular cylinder 7 c is preferably filled with alow-pressure gas. Alternatively a gas or air, for example at normalpressure, or a vacuum can be provided. The optical fiber 2 extends alongthe axis of the annular cylinder 7 c and is clamped between the fiberholders 6 a, 6 b in the region of the pressure sensor fiber 3. Anon-prestressed temperature sensor fiber 19 with a fiber Bragg grating20 is advantageously accommodated in a center cylinder 28 a. Aspreviously, pressure-tight fiber bushings 16, 17 are provided in thehousing walls 14, 15.

This embodiment is distinguished in that both supports 5 a, 5 b have acommon pressure member 7 c. The change in length of the annular cylinder7 c caused by internal pressure is converted by the rigid centercylinders 28 a, 28 b into a change in length of the pressure sensorfiber 3 which is of the sane (absolute) magnitude. The inner and outerwalls of the annular cylinder 7 c experience extensions of the samemagnitude when the cylinder covers 8 a, 8 b are sufficiently stiff.Moreover, it is possible to realize symmetrical loading of the annularcylinder 7 c by suitable selection of the wall thicknesses.

In order to compensate temperature-induced Bragg wavelengthdisplacements, the lengths and coefficients of expansion of the annularcylinder 7 c (L₁, α₁) and of the center cylinders 28 a (L₂′, α₁) and 28b (L₂″, α₂) are available as parameters. Of course, the center cylinders28 a, 28 b can also respectively be assembled from segments withdifferent coefficients of expansion α₁ and α₂, L₂′ and L₂″ respectivelydenoting the overall lengths of the center cylinder segments with α₁ andα₂ respectively. The thermal expansion of the center cylinders 28 a, 28b effects the desired relief of the fiber 3 with increasing temperature,and counteracts the thermal extension of the annular cylinder 7 c.Consequently, the equations G10 and G11 according to the invention arevalid in turn for the temperature compensation, and equation G9 holdsfor the special case that both center cylinders 28 a, 28 b have the samecoefficient of expansion α₂. In this embodiment, the pressuresensitivity is halved by comparison with FIGS. 1a, 1 b, since the oilpressure must lengthen the double-walled annular cylinder 7 c. Theprestressing can be selected to be the same as in the case of FIGS. 1a,1 b. It is advantageous that the sensor fiber 3, 19 is guided along thetransducer axis and requires no capillary 24 or similar protectionagainst the oil 11.

FIG. 3b shows a variant of the transducer 1 from FIG. 3a. Instead of theannular cylinder 7 c, the supports comprise a supporting tube 7 d, inparticular a hollow or supporting cylinder 7 d, which can be compressedby external pressure, and center cylinders 28 a, 28 b which can beextended by external pressure. In particular, the supporting cylinder 7d has end plunger faces 8 a, 8 b which are offset inwards in the regionof the axis and thereby form the center cylinders 28 a, 28 b. The fiberholders 6 a, 6 b are preferably fastened on the plunger faces 8 a, 8 bof the center cylinders 28 a, 28 b. The pressure exchange between thepressure cylinders 7 d, 28 a, 28 b and the medium 11 is performed, aspreviously, via an opening 18, which is protected, if appropriate, by adiaphragm 29 or the like. The supporting cylinder 76, which is closed ina pressure-tight fashion, is now tilled in the interior 21, preferablywith low-pressure gas, and remounted in the housing 12 via positioningelements 30. The arrangement of the fibers 2, 3, 19 is unchanged. Acapillary 24 can be provided to protect the fibers 2, 19 in the fluid 10or medium 11. Under pressure, the supporting cylinder 7 d is compressed,the center cylinders 28 a, 28 b are extended and the pressure sensorfiber 3 is relieved by the sum of the two deformations. As in FIG. 2a,the fiber prestressing is to be adapted to the desired pressure andtemperature ranges, and it is possible to realize a compressionarrangement 26, 27 (not illustrated) . The housing 12 together with thepositioning elements 30 can be omitted in a way similar to FIG. 1b. Thesame considerations as in the case of the transducer 1 according to FIG.3a hold for the temperature compensation.

An advantage of this transducer 1 is the pressure sensitivity, which isapproximately fourfold by comparison with FIG. 3a. This pressuresensitivity is the result, on the one hand, of the identical directionof the extension of the cylinders 7 d, 28 a, 28 b and, on the otherhand, of the approximately doubled elasticity of the supporting cylinder7 d by comparison with an annular cylinder 7 c of the same dimensions.

In summary, FIGS. 1-3 show exemplary embodiments of a fiber-optic sensor1 which is suitable, in particular, for pressure and temperaturemeasurement in oil drill holes. The sensor 1 comprises a transducer 1with fiber holders 6 a, 6 b for a pressure sensor fiber 3 which has atleast one fiber Bragg grating 4, the fiber holders 6 a, 6 b beingmounted on at least one support 5 a, 5 b. According to the invention, atleast one support 5 a, 5 b comprises a pressure member 7 a-7 d, 28 a, 28b which is suitable for converting an all round pressure of an ambientmedium 11 into a longitudinal elongation or compression of the pressuresensor fiber 3. A liquid, a gas a mixture of liquid and gas or finesand, inter alia, come into consideration as the medium 11. Inparticular, the pressure member is a pressure cylinder 7 a-7 d, 28 a, 28b, and has a cavity 9 with an opening 18 and a plunger face 8 a, 8 bwhich is operationally connected to a fiber holder 6 a, 6 b. Thetransducer 1 preferably comprises a pressure-insulated chamber 21, 24with pressure-tight fiber bushings 16, 17 for the pressure sensor fiber3, and just two supports (5 a, 5 b) are provided which consist of, orare assembled from materials with different coefficients of thermalexpansion α₁, α₂, a differential thermal expansion between thesesupports 5 a, 5 b counteracting a thermally induced displacement of theBragg wavelength λ_(B) of the pressure sensor fiber 3. The differentialthermal expansion between the two supports 5 a, 5 b, in particular thecylinders 7 a, 7 b or the center cylinders 28 a, 28 b, canadvantageously the continuously selected by virtue of the fact that atleast one of the supports 5 a, 5 b is assembled from at least twosegments 22, 23 with different coefficients of thermal expansion α₁, α₂and prescribable lengths L₂′, L₂″. A common sensor fiber 3, 19 can alsohave both the pressure sensor fiber 3 and the temperature sensor fiber19 with a fiber Bragg grating 20. Finally, also possible are transducers1 with many different forms and arrangements of pressure members,isotropic pressure being converted by means of said pressure membersinto longitudinal elongation or compression of the pressure sensor fiber3.

In the case of use in oil drill holes, all the transducers 1 exhibit thecommon problem of inward diffusion of gases, in particular of hydrogenand hydrocarbons, into the housing interior 21 or into the capillary 24.High hydrogen partial pressures of up to 20 bar can occur. Hydrogen inthe fibers 3, 19 causes optical losses and changes in refractive index,and thus interfering displacements of the Bragg wavelength λ_(B). Amethod according to the invention for protecting the sensor fibers 3, 19consists in coating the transducer surfaces not in contact with the oil11, in particular the housing interior 21 and/or the capillary 24,preferably with gold.

FIG. 4 shows exemplary embodiments relating to ferrules 32. Theanchoring of the pressure sensor fiber 3 in the ferrules 32 is verycritical, since the accuracy and long-term stability of the sensor 1 isimpaired by creeping of the fiber 3. In FIG. 4a, the fiber 3 is fusedwith a concentric glass capillary which can be fastened, for example, ina ferrule 32 by means of a bond 33. It appropriate, several glasscapillaries are interspliced. In FIG. 4b the fiber 3 is connected to theferrule 32 by glass solder 34. The ferrule 32 can also be open at leastone end in order to ensure that glass solder 34 is applied in acontrolled and uniform fashion, Specified in FIG. 4c is a clampingferrule 32 in the case of which a solid cylinder made from a soft metal35 is fixed on the fiber 3 by pressing on a hollow cylinder made fromhard metal 36. The pressing on is advantageously strongest in the middleof the ferrule, in order to achieve a longitudinal stress on the fiber 3which decreases outwards. In FIG. 4d, a length of fiber with a diametersimilar to that of the fiber core 38 a and the thicker cladding 38 b isinserted into the sensor fiber 3 via splices 37 and can be fixed veryeasily in the ferrule 32. Another solution (not illustrated) consists inbonding the pressure sensor fiber 3 into a V groove with the aid of anadhesive which is stable at high temperatures, for example one based onpolyimide. Finally, FIG. 4e shows how the preferably cylindrical orconical ferrules 32 can be fastened in or on ferrule holders 6 a, 6 b. Aparticularly space-saving solution consists in providing the fiberholders 6 a, 6 b themselves with a bore for accommodating the fiber 3,or fashioning them as ferrules 32.

FIG. 5 shows an overall design of a quasi-distributed pressure andtemperature sensor 48. A wavelength-division multiplex arrangement withseveral transducers 1 connected serially one behind another and operatedusing reflection is shown by way of example. The transducers 1 havedifferent Bragg wavelengths λ_(B) ^((i)) of their fiber Bragg gratingsfor pressure measurement 4 and temperature measurement 20. Thetransducers 1 are optically connected to a broadband light source 40 anda detection unit 43, preferably via a fiber coupler 42. In particular,feeder fibers 41 a-41 d are provided for bridging the optical linksbetween the active sensor optoelectronic system 40, 43, 47 and thetransducers 1. The detection unit 43 has a wavelength-divisiondemultiplexer 44 and a detector 45 which is typically connected to anelectronic evaluation system 47 via an electric signaling line 46. Thewavelength-division demultiplexer 44 can be a tunable spectral filter,for example a tunable Fabry-Perot interference filter, or a tunableacoustooptic modulator. The spectral width of the filter is to becomparable to that of the fiber Bragg gratings 4, 20, and preferablysmaller. The transducers 1 are detected individually by the photodiode45 by virtue of the fact that the filter is continuously tuned to theassociated Bragg wavelength λ_(B) ^((i)). The fibers 2, 3, 19, 41 a-41 dcan be of any desired type. The sensor fibers 3, 19 with the fiber Bragggratings 4, 20 are preferably monomode ones.

Fiber lasers doped with rare-earth elements, light-emitting diodes (LED)and superluminescent diodes (SLD) are particularly suitable as broadbandlight source 40 for the serial wavelength-division multiplexerarrangement 48 illustrated. The spectral emission range comprises theBragg wavelengths λ_(B) ^((i)), . . . λ_(B) ^((2n)) of the n transducers1 in the overall pressure and temperature tuning range. The individualtuning ranges are not to overlap. A central wavelength of 1550 nm isadvantageous for minimum losses in the case of large fiber links 41 a-41d in the range of several km. The typical spectral width is then ±25 nm.A maximum elongation of the temperature-compensated pressure sensorfiber Bragg grating 4 of 0.005 and a temperature range of 230° C. may beassumed. The associated tuning ranges are then 6 nm for the pressuremeasurement and 2.3 nm for the temperature measurement. With safetymargins, a transducer 1 therefore requires a wavelength window ofapproximately 10 nm, and the maximum number of pressure and temperaturetransducers 1 which can be subjected to wavelength-division multiplexingis limited to five. As an alternative or a supplement towavelength-division multiplexing, other multiplexing methods arepossible for increasing this number, for example time-divisionmultiplexing, or it is possible to use fiber-optic switches. It is alsopossible to realize in a simple way parallel or network-typeconfigurations of transducers 1 which are to be read out by reflectionand/or transmission.

Overall, the invention discloses a fiber Bragg grating sensor 1, 48 withdifferent transducers 1 for converting the hydrostatic pressure of aliquid or gaseous medium 11 into a longitudinal fiber elongation orfiber compression. Temperature-compensated transducers 1 with stableBragg wavelength λ_(B) can be realized by introducing a suitabletemperature dependence of the mechanical prestressing of the pressuresensor fiber 3 by selecting the materials, lengths and arrangements ofthe fiber holder supports 5 a, 5 b. As a supplement or alternative tothe passive temperature compensation, an active temperature measurementis possible with the aid of an additional fiber Bragg grating 20 andpressure signal correction. The transducers 1 are very suitable for useunder high pressures and at high temperatures.

LIST OF REFERENCE SYMBOLS  1 Fiber-optic pressure and temperature sensor(transducer)  2 Optical fiber  3, 19 Sensor fiber(s)  3 Sensor fibersegment, pressure sensor fiber  4 Fiber Bragg grating (for pressuremeasurement)  5a, 5b Supports  6a, 6b Fiber holders, ferrule holders 7a-7d Pressure members, pressure cylinders  7a Inner cylinder  7bReference cylinder; outer cylinder  7c Annular cylinder  7d Supportingcylinder  8a, 8b Plunger faces, end plates, cylinder covers  9 Cavity 10Fluid, silicone oil; medium 11 Medium 12 Transducer housing 13 Housingcylinder 14, 15 Transducer wall, housing wall 16, 17 Pressure-tightfiber bushings 18 Opening 19 Sensor fiber segment, temperature sensorfiber 20 Fiber Bragg grating (for temperature measurement) 21 Housinginterior, cylinder interior, low-pressure gas 22, 23 Outer cylindersegments 24 Capillary 25 Struts 26 Block 27 Bore 28a, 28b Holders,center cylinders 29 Pressure compensating element, diaphragm, bellows 30Positioning elements 31 Glass capillary (fused) 32 Ferrule 33 Bond 34Glass solder 35 Soft metal 36 Hard metal 37 Splices 38a Fiber core 38bFiber cladding 39 Fiber adapter 40 Broadband light source 41a-41d Feederfibers 42 Fiber coupler 43 Detection unit 44 Wavelength demultiplexer,(tunable) spectral filter 45 Detector, photodiode 46 Signaling line 47Electronic evaluation system 48 Overall sensor l Length of the pressuresensor fiber Δl Elongation of the pressure sensor fiber L₁ Length of thepressure cylinder L₂ Length of the reference cylinder L₂′, L₂″Sublengths of the reference cylinder ΔL₁ Extension of the pressurecylinder R_(i) Inside radius of the pressure cylinder R_(a) Outsideradius of the pressure cylinder E Young's modulus of elasticity of thepressure cylinder p Measuring pressure p_(max) Maximum measuringpressure T Temperature α₁, α₂, α_(f) Coefficient of thermal expansionΔε_(T) Thermally induced fiber elongation λ_(B), λ_(B) ^((i)) Braggwavelength (Δλ_(B))_(T), Bragg wavelength displacements (Δλ_(B))_(ε) i,,n Indices

What is claimed is:
 1. A fiber-optic sensor particularly suitable forpressure and temperature measurement in oil drill holes comprising: atransducer with fiber holders for a pressure sensor fiber: the fiberholders being mounted on at least one support, the at least one supportcomprises a pressure membrane for converting an all around pressure of amedium into a longitudinal elongation or compression of the pressuresensor fiber; wherein the pressure sensor fiber has at least one fiberBragg grating, said fiber being prestressed or precompressed in theregion of the at least one fiber Bragg grating.
 2. The fiber opticsensor in accordance with claim 1, wherein a) the transducer includes apressure-insulated chamber with pressure-tight fiber lead-ins for thepressure sensor fiber, and having b) exactly two supports which arecomposed of materials with different coefficients of thermal expansion(α₁, α₂), and wherein c) a differential thermal expansion between thesupports counteracts a thermally-induced shift in the Bragg wavelength(λ_(B)) of the pressure sensor fiber and d) at least one of the supportshas at least two segments with different coefficients of thermalexpansion and predefinable lengths.
 3. The fiber-optic sensor inaccordance with claim 1, wherein a) a sensor fiber comprises both thepressure sensor fiber and also a temperature sensor fiber and b) thetemperature sensor fiber has a fiber-integrated Bragg grating.
 4. Thefiber optic sensor in accordance with claim 1, wherein a) the pressurechamber is a pressure membrane, b) the pressure membrane having a cavitywith an opening and at least one piston surface and c) the pistonsurface being interactively connected with a fiber holder.
 5. The fiberoptic sensor in accordance with claim 4, wherein a) a first support inthe form of a pressure cylinder includes an inner cylinder which can beexpanded by internal pressure, b) a second support includes an outercylinder shielded from a surrounding medium, and wherein c) thecylinders are fixed to a transducer wall and encapsulated in atransducer casing.
 6. The fiber optic sensor in accordance with claim 4,wherein a) a first support in the form of a pressure cylinder includesan inner cylinder which can be expanded by internal pressure, b) asecond support includes an outer cylinder which can be compressed byexternal pressure, and c) the cylinders are fixed to a transducer wall.7. The fiber optic sensor in accordance with claim 4, wherein a) thesupports in the form of a pressure cylinder include a ring cylinderwhich can be expanded by internal pressure, b) the ring cylinder beingmounted in a transducer casing by means of positioning elements and c)the fiber holders are interactively connected with the ring cylinder byaxial cylinders.
 8. The fiber-optic sensor in accordance with claim 4,wherein a) the supports include a supporting cylinder which can beexpanded by external pressure and b) the fiber holders are fixed to thepiston surfaces of the axial cylinders.
 9. The fiber optic sensor inaccordance with claim 1, wherein a) several transducers of differentBragg wavelength (λ_(B) ^((i))) are optically connected with a wide-bandlight source and a detection unit by means of a fiber coupler and b) thedetection unit includes a wavelength demultiplexer and a detector whichis connected to evaluation electronics.
 10. The fiber optic sensor inaccordance with claim 1, wherein a) a block with a bore is provided toreceive the pressure sensor fiber, b) the pressure chamber membrane isfilled with a fluid, and c) the pressure-isolated chamber is filled witha low-pressure gas or a vacuum.
 11. The fiber optic sensor in accordancewith claim 10, wherein the fluid is silicone oil.